Can't copy and paste keyframes from another action
I'm not 100% sure, but mostly it works if I select all bones that have keyframes to copy, both dopesheet and 3d view.

However sometimes it's not working even do that, so that's why I said 100% sure.

I still facing this issue sometimes, but mostly works fine and some how it works after do something try to fix, but don't know what exactly fix this.